I bought a Stern Ali last year, and the first thing I noticed was the humming on power up. I have a lot of spare parts around, so I used some wires with alligator clips and jumpered a 5,000 uF 50Vdc capacitor in parallel with existing capacitors on the sound board and found the problem pretty quickly.
Looking closely at the boards in the head, I notice that all those filter capacitors are very old. I'm surprised any are still working.
When I get a chance I will replace them all with news ones. I have a bigger fix to perform first though, the last owner butchered the connectors to the AS 2518-18 rectifier/fuse board and soldered some wires directly to the board and it looks pretty sketchy.
For sure though, your humming will be traced to one or more bad capacitors.